Roly Poly Li
Richard Smalley aptly said, Energy is a $3 trillion a year enterprise, by far the biggest enterprise of humankind, even
bigger than agriculture. Fossil fuels currently supply most of the
world's energy needs. As fossil fuel supplies dwindle precariously and
environmental concern grows, there has arisen need for efficient,
clean, and renewable energy sources. Solar and wind-driven energy
generation exhibit potential for meeting future clean energy demands.
Such energy sources need to be coupled to electrical energy storage
devices so that there is no break in power supply from these otherwise
intermittent generators.
High-performance, rechargeable batteries are critical in alternate
energy source development. Such batteries are also needed for various
direct applications such as consumer electronics, communication
devices, medical implants and transportation. Search for suitable
materials with high-energy efficiency has accelerated the rapid
development in metal hydride and lithium ion chemistry.
Lithium ion batteries are currently one of the most promising types
of battery for conjunction with alternate energy systems and are
already used to a large extent in portable electronics due to the
following known advantages:
- High energy-per-weight ratio
- Cheaper in terms of energy delivered per unit of weight than
alternative types of battery such as nickel-metal-hydride (NiMH) and
nickel-cadmium (Ni-Cd) types.
- Lower self discharge rate than other types of rechargeable
batteries. NiMH and NiCd batteries can lose anywhere from 1-5% of their
charge per day, (depending on the storage temperature) even during
storage. Lithium-ion batteries will retain most of their charge even
after months of storage.
Fig. 1: Energy density comparison [1]
The conventional lithium ion battery contains some form of liquid
electrolyte, usually a lithium salt in an organic solvent. Use of
liquid electrolytes presents a few drawbacks, the most compelling of
them being inconvenience of design and use. These electrolytes are
toxic or flammable, and can result in damaging leaks. This site contains a creepy U-Tube presentation of a laptop bursting into flames due to its lithium ion battery electrolyte leak.
Fig. 2: Flaming laptop
Use of solid state electrolytes can alleviate many of the problems
associated with liquid electrolytes. Many dielectric materials, such as
polymers, glasses, ceramics, and their combination, have been studied
for use as solid electrolytes. Among those materials, polymers have
received considerable attention in the last two decades because of
their low density, ease of manufacturing and capacity to accommodate
volume changes as compared to a true rigid, inorganic solid electrolyte
[2]. Apart from advantages of easy handling and workability,
the advantages of lithium polymer batteries (Li-poly) over conventional
lithium-ion design include lower cost of manufacturing and higher
resistance to physical damage.
Solid polymer electrolytes however, exhibit lower ionic conductivity
than liquid electrolytes, but are less reactive with lithium and
therefore increase the safety of the batteries. Such polymer
electrolytes can be used as the electrolyte or the separator or both.
The ability of the polymer electrolyte to form thin, flexible and
transparent films increases the design possibilities for thin
batteries. The development of all-solid lithium polymer batteries has
been so far hindered by the lack of a suitable polymer electrolyte,
capable of fulfilling the stringent operational requirements in terms
of ion transport properties and compatibility with the electrode
materials.
Ionic conductivity in polymer electrolytes has been reported to
occur in a manner somewhat analogous to gas diffusion through polymer
membranes. Segmental motion of the polymer chains continuously creates
free volume into which the ions migrate, and this process allows them
to progress across the electrolyte [3]. Attention for the
past few years has concentrated on the synthesis of new polymeric
materials with low glass transition temperature and hence high levels
of segmental motion in order to increase the conductivity [4].
The complexes of crystalline poly(ethylene oxide) (PEO) with lithium
salts have been considered suitable electrolytes for Li-poly batteries
as early as 1973 [5]. It was recognized that this type of lithium conducting materials show specific conductivities in the order of 10-8-10-7
S/cm. However, for application as energy storage devices to be used in
conjunction with solar and wind generators, conductivities approaching
at least 10-3 S/cm must be reached. PEO-based solid polymer
electrolytes have so far not reached a satisfactory status in the
lithium battery technology due to the following reasons [6]:
1. Small but sure reactivity with the lithium metal electrode, which affects the battery cyclability,
2. Low lithium ion transference number, which affects the kinetics
of the electrochemical process and, thus, the rate capability of the
battery, and
3. Low ionic conductivity at ambient temperatures, with limits the temperature range of utilization of the battery.
Amorphous PEO salt complexes have been reported to show superior
ionic conductivity when compared to crystalline PEO salt complexes,
probably due to the segmented motion of the polymeric chain during ion
transport. The lower glass transition temperature of amorphous PEO
compared to crystalline polymer results in a higher ionic mobility [7].
Various approaches have been studied to improve the ion transport properties of solid polymer electrolyte. Some of them are:
1. Blends: PEO is blended with iorganic supports or macromolecules that inhibit crystallinity of PEO [8]
2. Hybrids: Inorganic network growth in organic polymers have been found to marginally improve ionic mobility [9]
3. Gels: The polymer electrolyte is in the form of a gel that entraps the liquid electrolyte [10]
4. Plasticized
electrolyte: The addition of a liquid plasticizer such as propylene
carbonate has been found to improve the ionic conductivity to 10-4 Scm-1. However addition of plasticizer degraded the mechanical stability of the PEO [11];
the gain in conductivity is adversely accompanied by a loss of the
solid-state configuration and by a loss of the compatibility with the
lithium electrode, i.e., by a loss of the most important intrinsic
features of the polymer electrolyte.
More recently, there has been increasing interest in nanocomposite
polymeric materials because of their high ionic conductivities and
superior mechanical stability over pure polymer electrolytes [12]. The basic method of composite polymer electrolyte formation is the dispersion of inorganic oxides like TiO2, Al2O3,
etc., into a polymer-salt electrolyte solid matrix . It has been
reported that the dispersed nanoceramics promote shielding and
scavenging actions that modify or even stop the growth of the
passivation layer on the surface of lithium, greatly improving the
stability of the lithium interface with consequent enhancement of the
efficiency of the lithium cyclability.
Ongoing research in the field of solid state lithium ion battery
electrolytes include modifying the polymer structure to promote ion
mobility and incorporation of nanostructured inorganic inclusions to
facilitate ion transport.
References
1. J.-M. Tarascon and M. Armand Nature 414, 359-367(15 November 2001)
2. Kumar B., Scanlon L.G.: Journal of Electroceramics. 2000, vol. 5(2), p. 127
3. Bruce, P. G. Solid State Electrochemistry (Cambridge Univ. Press, Cambridge, 1995)
4. Christie, A.M., Lilley S.J., Staunton E., Andreev Y.G., Bruce P.G., Nature. 2005, vol. 433, p. 50
5. M.B. Armand, in: Fast Ion Transport in Solids, W. Van Gool (Ed.) Elsevier, North Holland, 1973.
6. Croce F., Serraino Fiory F.,Persi L.,Scrosati B.:Electrochemical and Solid-State Letters. 2001, vol. 4(8), p A121
7. Bruce P.G., Vincent C.A., J. Chem. Soc. Faraday Trans. 1993, vol. 89, p 3187
8. Bakker A., Lindgren J., Hermansson K.: Polymer. 1996, vol. 37, p. 1871
9. Zoppi R.A., Polo Fonsesca C., De Paoli M.-A., Nunes S.P.: Acta Polym. 1997, vol. 48, p 131
10. Florjanczyk Z., Bzducha W., Langwald N., Dygas J.R., Kork F., Misztal-Faraj B.: Electrochim. Acta. 2000, vol. 45, p 3563
11. Ishikawa M., Morita M., Iihara M., Matsuda. Y.: J. Electrochem. Soc. 1994, vol. 141, p 1730.
12. Croce F.,. Appetechi G.B, Persi L., Scrosati B,: Nature. 1995, vol. 373 p. 557.
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